Various drive pulses are required to drive a solid image sensor such as CCD (Charge Coupled Device) or CMOS (Complementary Metal-Oxide Semiconductor). To generate good image signals, it is necessary to generate the drive pulses in phases in a certain relation. This holds true for sampling process to prevent noise in image signals. Noise cannot be reduced properly unless sampling pulses are adjusted in phase in a certain relation (disclosed in Japanese Patent Application Publication No. 2009-081840 (Reference 1) and No. 2010-062902, for example).
Reference 1 discloses for example a technique to properly adjust CCD drive pulses and sampling pulses for correlated double sampling in phase by opening a lens or elongating the read interval of a CCD to attain sufficient exposure time so that CCD output reaches a defined saturation level. An automatic gain controller is set to make the CCD saturation output be the full scale of AD conversion, and the ideal phase relation of horizontal drive pulses are calculated to set an a phase adjusting circuit. A range in which signals are output from the CCD is determined by separately scanning sampling pulses SHP/SHD in the initial state to store a maximum output point.
Meanwhile, aperture delay is a problem in driving a CCD in an image reader. FIG. 2 is a timing chart for general aperture delay. Aperture delay refers to time Tad from when a sampling edge is supplied to an analog front end circuit (hereinafter, AFE circuit) to when a sample of an input signal is actually held. As shown in FIG. 2, the level of an input signal from the CCD is broadly divided to three levels, a reset level 31, a pedestal (black) level 32, and an image signal level 33. The AFE circuit samples and AD converts the input signal in the image level 33 for output.
To increase dynamic range and reduce jitters, it is preferable to sample the signal in a stable area of the image level close to the reset level.
However, while the CCD processes at high speed, this stable area is narrowed so that it is difficult to sample the signal in a stable image level with a large aperture delay with a fluctuation in power supply voltage, temperature, and process fluctuates taken into account.